The Nematoid order, or thread-worms, that live in the muscles of men and animals, are long, smooth, and cylindrical, with a structureless skin covering layers of longitudinal and circular fibres, by means of which they can stretch and contract themselves. They are generally pointed at both ends with a mouth at one extremity and an orifice at the other. The Filariæ are slender, sometimes of great length, as the Guinea worm, which varies in length from six inches to two, eight, or even twelve feet. In Persia they are believed to be introduced into the system by drinking water in which their eggs have been deposited. This worm may grow in the muscles of a man to the size of five or six feet without giving much annoyance, but when its head bores through the skin it produces a painful sore unless extracted. In Persia, where the worm is common, the natives seize it by the head, draw it carefully out, and wind it round a bit of wood, an operation which may require several days to accomplish. It has a numerous viviparous progeny, which come out through the mouth. There are certain very small slender species of Filaria which attack the eyes both of men and horses; some bury themselves close to the eye, and a very minute kind enters the ball itself. The Ascaris lumbricoïdes, a common intestinal thread-worm of the hog, ox, and the human race, is sometimes of great length. The sexes are distinct, and their fertility enormous. The ovaries are two tubes sometimes several feet long, in each of which the eggs are arranged in whorls round a central stem, like the flowers of a plantago. By counting the number of microscopic eggs in a whorl, and the number of whorls, Dr. Eschricht ascertained that in a full grown female the average number of eggs amounted to sixty-four millions. In this species of worm the embryo is not developed from the egg while within the victim, so that most of the eggs perish. Different species of Anguillulæ, which are minute eel-like worms slender as a hair, inhabit the alimentary canal of fresh-water snails, frogs, and fishes, but many species are not parasitic. These are often united in swarming masses that nestle in mud, wet moss, wet earth, and aquatic plants. One species causes the cockle in wheat, appearing like a living tuft of white wool in the blackened grains. They appear in sour paste and in other decomposing substances, and are so tenacious of life that, after being completely dried for months, and apparently dead, they revive on being moistened. _Turbellariæ._